1. Field of the Invention
The present invention relates to a slide rail device for making an adjustment to the position of a vehicle seat in the forward/rearward direction.
2. Description of Related Art
A typical slide rail device (slide rail assembly) is configured of a lower rail that is fixed to a vehicle floor, an upper rail that is fixed to a vehicle seat and engaged with the lower rail to be freely slidable thereon, and balls (steel balls) for reducing frictional resistance that are installed between the upper rail and the lower rail. The upper rail and the lower rail are each made of a metallic material (usually an iron-based material) having a uniform cross section. Such a slide rail device is disclosed in Japanese Patent Publication No. 4,230,945.
In this known slide rail device, application of a force on the upper rail and the lower rail therebetween that is produced by the weight of someone (a driver or a passenger) sitting in the vehicle seat causes relatively large forces that exert on the rails at points of contact with the balls. As a consequence, sometimes indentations are made on the rails via the balls. Since such indentations obviously deteriorate the rollability of the balls, it is desirable that the rails be formed into a shape preventing such indentations from being made on the rails as much as possible. For this reason, the radius of the balls has been set to be identical to the radius of curvature of the ball contact portions of each rail in conventional slide rail devices. Namely, if the radius of the balls is identical to the radius of curvature of the ball contact portions of each rail, the surface pressure exerted on the rails is at a geometric minimum, so that the possibility of indentations being made on the rails via the balls is low. However, the inventor of the present invention has found that, if the radius of the balls is made to be identical in design to the radius of curvature of the ball contact portions of each rail, the rollability of the balls sometimes deteriorates regardless of the presence or absence of indentations on the rails (aside from the problem of indentations), to thereby make the sliding operation of the upper rail with respect to the lower rail unstable.